Display devices, including visual display devices, play an important part in modern technology and commercial devices available to the public. Typical display devices are cathode ray tubes for use in television sets, tubes used as monitors or in projection television tubes. Also of commercial importance are x-ray imaging devices in which typically x-ray radiation is converted into visible radiation. Phosphors are used to convert various kinds of energy, particularly electromagnetic radiation energy and electron beam energy into radiation in the visible region or radiation region directly adjacent to the visible region such as the infrared region or ultraviolet region.
Cathode ray tubes (CRTs) are especially useful in display devices. They are extensively used in direct view and projection television sets, monitors for computer terminals, television and avionics systems, etc. In many applications (such as projection tubes), high image brightness is required which can only be obtained by the use of a very high power density electron beam. Such high power densities often degrade conventional cathode ray tubes and therefore limit the lifetime of high intensity cathode ray tubes.
A significant advance in the development of high intensity cathode ray tubes was the discovery that certain luminescent epitaxial garnet films on single crystal substrates could withstand much higher power densities than with powder phosphors without tube degradation (see, for example, J. M. Robertson et al., Applied Physics Letters, 37(5), pp. 471-472, Sept. 1, 1980). Several systems were examined using yttrium aluminum garnet as the substrate and various activators in yttrium aluminum garnet as the epitaxial layers. The activators examined were Tb, Eu, Pr, Tm and Ce. The epitaxial layers were grown by liquid phase epitaxy using a PbO--B.sub.2 O.sub.3 flux.
These types of fluorescent screens did indeed withstand much higher electron power densities than conventional screens and maintained their performance without long term degradation. Therefore, it is highly desirable to find single crystal phosphors with high efficiency, high saturation levels, and colors comparable with those colors used in color television tubes. Phosphors with high efficiency are desirable because less exciting energy is needed to produce a useful phosphor output. High saturation levels produce phosphors with high dynamic ranges without saturation effects.
A number of powder phosphors have been made by solid state reaction. Some have been described in a paper entitled "Solid Solutions in the Y.sub.2 SiO.sub.5 --Y.sub.2 GeO.sub.5 System" by N. A. Toropor et al, Izvestiya Akademii Nauk SSSR, Neorpanicheskie Materialy, Vol. 5, No. 2, pp. 321-324 (February 1969). A paper by T. E. Peters entitled "Cathodoluminescent Ln.sub.y (SiO.sub.2).sub.x : Tb Phosphors "Journal of the Electrochemical Society: Solid /State Science, 116, No. 7, pp. 985-989 (July 1969), discloses powder luminescence in a number of powders including Tb doped Y.sub.2 SiO.sub.5. Crystallographic data on single crystal Y.sub.2 SiO.sub.5 is given in a paper by L. A. Harris et al. entitled "Crystallographic Data for Er.sub.2 SiO.sub.5 and Y.sub.2 SiO.sub.5 ", American Mineralogist, 50, pp. 1493-1495 (1965).
Synthesis of single crystals of Y.sub.2 SiO.sub.5 is described in number of references. A. M. Morozov et al, in a paper published in Optical Spectroscopy, Vol. 41, No. 6, pp. 641-642 (December 1976) describes synthesis of small crystallites of Y.sub.2 SiO.sub.5 by the Verneuil method and measurement of the luminescence and stimulated emission of holmium doped Y.sub.2 SiO.sub.5. Also described in the literature is certain crystallographic measurements on single crystals of Y.sub.2 SiO.sub.5 made by hydrothermal crystallization in the system K.sub.2 O--Y.sub.2 O.sub.3 --SiO.sub.2 --H.sub.2 O at 450.degree. C. and about 1500 atm.